What is your definition of Leucistic? By mine, Ball Pythons have yet to produce a Leucistic. It all has to do with chromatophores.
Amelanistic = no melanin, Achromatophoric animals don't have chromatophores or they are not functioning. Chromatophores are what bring melanin, xanthin, and erythin together and the product of iridiphores.

A Leucistic animal is Achromatophoric. It has all the ingredients for its color, but since the chromatophores aren't working, it appears white. Each of the White Ball Pythons can/does show some pigment on their body (shades or yellow, pink, grey, etc.). The Black-eyed or Super Fire BP is the closest (visually) as some have no color, but most have yellow scales in random locations. I believe the Super Lesser also produces some that are pure white.

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